(Supported by NSF DMB 8916315 to C.A. Mannella) This is the "umbrella" project for study of the mitochondrion using electron tomography. The goals are to determine the interactions between the inner and outer mitochondrial membranes, the continuity between internal compartments, and the interactions between mitochondria and other cytoplasmic structures under various metabolic conditions. This subproject is driving the TRD subprojects involving development of cryo-EM techniques for mitochondria, as well as CS subprojects involving collaborations with outside investigators. Aspects of this work not reported in other subprojects is reported here. Several double-tilt series of plastic-embedded Neurospora crassa mitochondria in 0.25(m thick sections were recorded on the IVEM, and reconstructions are being made. A high school student, Alice Yip, is assisting with this work, and the CS subproject "Tomography of yeast mitochondria that have undergone permeability transition", for the Westinghouse Science Competition. The results of all the subprojects involving mitochondria are now fitting together to give new insight into mitochondria structure and function, and a major paper is being organized. The outlines of this were presented as the keynote talk at the Albany Conference on Frontiers of Mitochondrial Research in September. Mannella, C.A. (late 1997) On the structure and gating mechanism of the mitochondrial channel, VDAC. J. Bioengineering and Biomembranes 29:525-531 Mannella, C.A., K. Buttle, Tessitore, K., B.K. Rath, C. Hsieh, D. D'Arcangelis, M. Marko. (1998) Electron microscopic tomography of cellular organelles: Chemical fixation versus Cryo-substitution of rat-liver mitochondria. Microscopy and Microanalysis 4:430-431. Mannella, C. M. 1998. Conformational changes in the mitochondrial channel protein VDAC and their functional implications. J. Structural Biology., 121:207-218. Mannella, C.A., Buttle, K., Pfeiffer, B..R., Rath, B.K., Marko, M. (1998) Mitochondrial design: implications for the structural basis of the permeability transition. Proc. Albany Conference on Frontiers of Mitochondrial Research.